Refrigeration systems are used to cool an enclosed area, such as a house or a food storage case, by removing heat from the air in the enclosed area and releasing the heat outside the enclosed area. In a typical refrigeration system, a highly-pressurized liquid refrigerant, such as freon, is introduced into a low-pressure environment in the enclosed area and allowed to evaporate, or "boil," thereby drawing heat from the enclosed area. The superheated vapor is then compressed and condensed outside the refrigerated area, releasing the heat removed from the refrigerated area.
FIG. 1 shows a typical refrigeration system 10 used to cool a food storage case 12. Highly pressurized liquid refrigerant is released from a receiver vessel 14 into a pipe 16, or it may bypass the receiver vessel 14 through a bypass line 15. A valve 18, such as an expansion valve, provides an orifice in the pipe 16 that opens and closes to restrict the flow of liquid refrigerant, creating a high pressure region between the valve 18 and the receiver vessel 14. As the refrigerant passes through the expansion valve 18, it encounters lower pressure in the evaporator coil 22 and begins to evaporate. Because evaporation is an endothermic process, the refrigerant pulls heat from the surroundings of the evaporator coil 22 (i.e., the food display case) as it evaporates. A compressor system 20 provides suction pressure at the coil outlet that, when properly controlled, helps ensure that the last bit of liquid refrigerant evaporates just before it leaves the evaporator coil 22 (i.e., that the entire coil 22 is used to remove heat from the case 12). The compressor system 20 then pressurizes the vaporous refrigerant and discharges it to a condenser coil 28, where the refrigerant condenses. Because condensation is an exothermic process, the refrigerant releases the heat removed from the food storage case 12 as the refrigerant returns to the liquid state.
In general, a refrigeration system designer strives to accomplish several important goals, including the following. First, the system should maintain the temperature of the refrigerated area at a substantially constant level. Second, the system should use the entire length of the evaporator coil to boil refrigerant (i.e., maximize coil efficiency) and, at the same time, to ensure that no liquid refrigerant enters the compressor system. Third, the system should maintain suction pressure at the highest possible level.
In the system 10 of FIG. 1, the expansion valve 18 attempts to maximize the efficiency of the evaporator coil 22 by maintaining a predetermined temperature difference across the coil 22. The valve 18 measures coil temperature (or inlet pressure) at the coil inlet 22a, and a temperature transducer 24 measures coil temperature at the coil outlet 22b. If the temperature difference across the coil 22 is too high, all of the refrigerant is evaporating before it reaches the end of the coil 22, and thus the coil 22 is not operating efficiently. If the temperature difference is too low, liquid refrigerant is escaping the coil 22, thereby wasting refrigeration potential, and possibly entering the compressor 20. The system 10 attempts to stabilize the temperature difference across the coil 22 by increasing or decreasing the duty cycle of the valve 18 to accordingly adjust the flow of refrigerant into the evaporator 22. When the refrigerant is boiling too rapidly, the expansion valve's duty cycle is increased to allow more refrigerant to enter the coil 22; and when liquid refrigerant is escaping the coil 22, the duty cycle of the expansion valve 18 is reduced to reduce the flow of refrigerant into the coil 22. The valve 18 may be a spring-loaded valve, a mechanical expansion valve, or an electronic expansion valve, all of which are known in the art and are not described in detail here.
Suction pressure, and therefore evaporator temperature and case temperature, are controlled by the compressor system 20. When the case temperature deviates from a predetermined value, the compressor system 20 may increase or decrease suction pressure to alter the amount of refrigerant that evaporates, and thus the amount of heat removed, in the evaporator coil 22. Because the compressor system 20 must expend energy to pressurize the refrigerant flowing into the condenser coil 28, the compressor system 20 should maintain the highest possible suction pressure. As suction pressure falls, the compressor system 20 must work harder to maintain discharge pressure to the condenser coil 28, and as suction pressure rises, the compressor system 20 may reduce its work and still maintain discharge pressure. The compressor system 20 also affects case temperature by cutting on and off at appropriate times. The compressor system 20 is controlled by a controller 26 that receives as input the case temperature and the output of a suction pressure transducer 29.
The compressor system may service several food storage cases and thus several evaporator coils. In this situation, each evaporator coil requires a separate expansion valve, but all coils are subject to the same suction pressure. Furthermore, the compressor may be replaced by a group of compressors which are operated in tandem to enhance the refrigeration system.